The present invention relates to a harmonic and pulsation reducing circuit in a multiplex polyphase rectifier circuit with an interphase reactor or an inverter with an interphase reactor and more particularly, relates to an improvement for reducing harmonics in the current on AC side and pulsations in the voltage on DC side of a multiplex polyphase rectifier or of an inverter, respectively having a multiplex polyphase rectifier circuit.
As rectifiers have become more popular and the capacity thereof has increased in recent years, harmonics from a rectifier causes often a serious problem in electrical power transmission or distribution system. Although various countermeasures to solve the problem by increasing the number of phases or pulses of a rectifier or by using a filter have been proposed, they are not always desirable as they tend to push cost considerably.
Explanation is given first to a double three-phase 12-pulse rectifier with an interphase reactor which comprises star connections and delta connections in combination.
FIG. 1 is a circuit diagram of a well-known double three-phase 12-pulse rectifier of such a type. As shown in the figure, a three-phase bridge rectifier circuit 1 having 6 thyristors T.sub.1 to T.sub.6 as rectifying elements is connected to one of the secondary windings in star connection of a transformer, while a three-phase bridge rectifier circuit 3 having 6 thyristors T.sub.7 to T.sub.12 as the rectifying elements is similarly connected to the other secondary winding 4 in delta connection of the transformer. Both three-phase bridge rectifier circuit 1 and 3 are connected to each other via the end portions A.sub.1, A'.sub.1 of an interphase reactor 5a to supply DC current to a load 7 which is connected via a smoothing reactor 6 to the central point A of the interphase reactor 5a. The reference numeral 8 denotes a primary winding of the transformer.
The waveforms of the voltage e.sub.d between terminals A.sub.0 and B of the voltage .nu.m which is to be applied to the interphase reactor 5a at the normal operation of the double three-phase pulse rectifier may vary depending on the phase-control angles .alpha. of the thyristors T.sub.1 to T.sub.12. In short, when the control angle .alpha. is extremely small, the waveforms of the voltages e.sub.d , .nu.m become as shown in FIG. 2(a), (c): the voltage e.sub.d includes the pulsation of 12f (f=power source frequency) shown in FIG. 2(b) and the voltage .nu.m becomes substantially triangular in waveforms. When the control angle .alpha. is large or close to 90.degree., the waveforms of the voltages e.sub.d, .nu.m becomes as shown in FIG. 3(a) and (b). That is, the voltage e.sub.d is serrated and includes the pulsation of 12f while the voltage .nu.m becomes substantially rectangular.
The pulsation contained in the voltage e.sub.d , for instance, of 12f in this case can be reduced in such a prior art only by increasing the number of phases or by using quadruple star zigzag connections as mentioned above.